You can do manually do a cross resolve on the editor timeline by working with two video tracks. In the middle of any track you see that line representing the opacity. All you need to so is keyframe that.

When you want a cross fade for e.g. 15 seconds to half and than back to full you go to sec. 15, make a keyframe by pressing STRG+left mouse, drag that down to the desired opacity, then go to sec 30 and do the same with that point dragged up to full opacity.